FR3013047A1 - PROCESS FOR SYNTHESIZING ESTERS - Google Patents

Abstract

According to one embodiment, the invention relates to a process for synthesizing a second ester from a first ester, the process comprising the following steps: a) bringing a first ester and a catalyst into contact with each other and dihydrogen to obtain a first alcohol and a second alcohol; b) extracting said second alcohol from the reaction medium; c) reacting said first alcohol with a catalyst to obtain a second ester and dihydrogen; and d) recycling the dihydrogen obtained in step c) by introducing it in step a).

Description

TECHNICAL FIELD The invention relates to a process for synthesizing esters from biosourced starting products by dehydrogenating coupling in the presence of a catalyst. The starting materials may be biosourced esters such as fatty acid methyl esters (FAME) from oleaginous plants, for example methyl oleate. PRIOR ART Many esters, and especially ethyl acetate, are synthesized on an industrial scale by using starting materials of fossil origin (ethylene in the case of ethyl acetate) via processes multi-step. The world market for ethyl acetate was 2.5 million tons / year in 2008. It is known to use a ruthenium-based catalyst to carry out the dehydrogenating coupling of ethanol using, for example, carbonylchlorohydrido [bis (2-diphenylphosphinoethyl) amino] ruthenium (II), of formula A, (CAS: 1295649-40-9), Trade Name: Ru-MACHO, or D (see below) (see M. Nielsen, H. Junge, A. Kammer and M.Beller, Angew Chem Int.Ed., 2012, 51, 5711-5713) or transRuC12 (PPh3) [PyCH2NH (CH2) 2PPh2], of formula B (cf. D. Spasyuk and D. Gusev, Organometallics, 2012, 31, 5239-5242). These catalysts A and B, however, require the presence of tBuOK or EtONa, to be active. Catalyst D requires a long reaction time and a high catalytic charge to obtain yields of up to 90% ester. ## STR2 ## wherein: ## STR2 ## Another catalyst used for this same reaction is the catalyst carbonylhydrido (tetrahydroborato) [bis (2-diphenylphosphinoethyl) amino] ruthenium (II), of formula C (CAS: 1295649-41-0), Trade Name: Ru-MACHO -BH. This reaction is described in the patent application WO2012 / 144650 where this synthesis requires the presence of a ketone, for example 3-pentanone. It is also known to use a ruthenium-based catalyst for carrying out the dehydrogenating coupling of butanol using, for example, trans-RuH 2 (CO) 2 HN (C 2 H 4 P 2 O 2) 2]. of Formula D (Bertoli M., A. Choualeb, AJ Lough, B. Moore, D. Spasyuk and D. Gusev, Organometallics, 2011, 30, 3479-3482) or [RuH (PNN -) (CO) ], of formula E (see J. Zhang, G. Leitus, Y. Ben-David and D. Milstein, J.

Am. Chem. Soc., 2005, 127, 10840-10841) in the presence of solvent and in the absence of base and hydrogen acceptor. However, these catalysts require long reaction times and high catalyst loads to obtain yields of up to 90% ester. However, the introduction of such processes on an industrial scale has many disadvantages. One of these disadvantages is the need to carry out purification steps to isolate the products of the reaction. Another problem is the use of organic products in addition to the products of departure. The use of such products, such as organic solvents, significantly increases the environmental impact of such syntheses, which is of course to be avoided. It has now been realized a process that solves these problems and thus offers a realistic alternative to industrial processes of ester synthesis using fossil resources. This process makes it possible to combine high yields with simplified synthesis and purification steps at the level of implementation.

DESCRIPTION OF THE INVENTION According to one embodiment, the invention relates to a process for synthesizing a second ester from a first ester, the process comprising the following steps: a) bringing a first ester into contact with a catalyst of formula 1 and dihydrogen to obtain a first alcohol and a second alcohol; b) separating said second alcohol from the reaction medium; c) reacting, for example by heating, said first alcohol with said catalyst of formula 1 to obtain a second ester and dihydrogen; and d) recycling the dihydrogen obtained in step c) by introducing it in step a); said formula 1 being: ## STR1 ## wherein R are groups, identical or different, selected from the group consisting of cyclohexyl, phenyl, isopropyl and ethyl; where Y is a PR'3 phosphine, a CO radical or a hydrogen atom, R ', which may be identical or different, and are C1-C12 alkyl or C6-C12 aryl groups; and wherein Z is a hydrogen atom, an HBH3 group or a halogen atom. According to a preferred embodiment of the invention, step c) is carried out without the addition of a hydrogen acceptor. Advantageously, steps a) to c) are carried out without additional addition of a hydrogen acceptor. According to a preferred embodiment of the invention, step c) is carried out without the addition of a solvent. Advantageously, steps a) to c) are carried out without addition of solvent. According to a preferred embodiment of the invention, step c) is carried out without addition of base. Advantageously, steps a) to c) are carried out without addition of base. The term "hydrogen acceptor" refers to organic compounds that are capable of reacting with molecular hydrogen, H2, to form a novel compound under the reaction conditions of ester synthesis. In particular, it may be compounds such as ketones, aldehydes and alkenes. The absence of compounds that can react with molecular hydrogen to absorb it in step c) is particularly advantageous because it allows in particular the production of hydrogen gas H2 which is easily isolated from the reaction medium and can therefore be used subsequently. for example for step a). In addition, this avoids the stoichiometric formation of the hydrogen acceptor hydrogenation product, facilitating the downstream purification steps. Thus, the process according to the invention also makes it possible to obtain gaseous molecular hydrogen. It separates from the reaction medium by simple phase separation and can be evacuated and / or collected directly and then reused in step a) of the process. This process is therefore particularly suitable for an industrial production device because it allows simple and effective recycling of the by-products of the reaction, thus limiting the need for additional hydrogen for the first step.

The term "base" refers to a compound capable of capturing one or more proton (s). In the context of the invention, the term "base" refers in particular to bases such as sodium hydroxide or alkoxylated alkali metal salts such as EtONa, MeOnA or tBuOK. In a preferred manner, the reaction is carried out in the absence of a solvent.

The term solvent refers to a substance, liquid at its temperature of use, which has the property of dissolving, diluting or extracting alcohols and optionally the catalyst without chemically modifying them and without itself changing. In the context of the invention, the term "solvent" refers especially to ketones, such as 3-pentanone, acetone or cyclohexanone. It also denotes aromatic or aliphatic hydrocarbon compounds, optionally halogenated, ethers and alcohols. This solvent in the absence of which the reaction is carried out can obviously also be a hydrogen acceptor and / or a base.

The term "absence of" is used in its normal sense which implies the absence in the initial reaction mixture of a sufficient amount of the compound to play an effective role in the reaction as well as the absence of external addition of this compound. during the reaction. For example, the presence in the reaction medium of a small amount (for example in the form of a trace) of a hydrogen acceptor, a base or a solvent will not influence the reaction substantially. Thus, the absence of a solvent implies the absence of an amount sufficient to dissolve / dilute / extract the starting product (s) (ie the ester (s) and the alcohol (s)) as well as the catalyst. For a solvent this amount is generally greater than the numbers of moles of reagents, that is to say that the solvent is generally present in the reaction medium in a molar concentration greater than or equal to 50%. Preferably, the expression "absence of" implies a molar concentration of said compound of less than 10%, and more particularly less than 5%, or even its total absence, that is to say less than 0.001%. Preferably, in formula 1 of the catalyst, Z is not a halogen atom.

Preferably, the group R 'of formula 1 is a methyl, ethyl, isopropyl or phenyl group. It is also preferred that the R 'groups be identical. Advantageously, the four R groups are identical. Preferably R is a cyclohexyl or isopropyl group.

According to a particular embodiment of the invention, the first ester, wherein the starting ester, has the following formula: ## STR1 ## where R 1 is a linear or branched alkyl group, from C 2 to O 2, and R 2 is an alkyl group of Ci to C6. R1 and R2 may also include unsaturations or functionalizations such as hydroxyl groups, amines, and ketones: The starting ester, or first ester, may also have the following formula: R3 R3 R3 R3 Where R3 are alkyl groups of C8 to C22, identical or different, linear or branched. R3 can also comprise unsaturations or functionalizations such as, for example, at least one hydroxyl, amine or ketone group. For example, the first ester may be a fatty acid methyl ester (FAME) such as methyl oleate. Preferably, the catalyst charge used in the process according to the invention, and more particularly in steps a) and c), is less than 10000 ppm, more particularly less than 1000 ppm, even more particularly less than 500 ppm. This charge may be for example about 50 ± 10 ppm. Preferably, the catalyst charge used in the process according to the invention is chosen from a range of from 10000 ppm to 1 ppm, more particularly from 1000 ppm to 10 ppm and even more particularly from 500 ppm to 50 ppm. This charge may be for example about 225 ± 10 ppm. The proportions above are given in relation to the total mass of the starting materials. Preferably, step a) is carried out under hydrogen gas pressure, said hydrogen pressure being able to be chosen in the range from 10 to 50 bar, preferably from 20 to 30 bar, for example at a pressure of about 20.degree. Advantageously, the isolation step b) is carried out by known separation techniques such as vacuum evaporation or aqueous extraction. The product of the reaction generally comprising a second, undesired alcohol, is advantageously extracted from the reaction medium. Thus the subsequent esterification reaction allows to couple 2 molecules of the same alcohol to create a new ester, different from the original one. Preferably, step c) comprises heating the reaction medium and is carried out at a temperature chosen from a temperature range of from 200 ° C. to 15 ° C., more particularly from 150 ° C. to 40 ° C. and even more particularly from 130 ° C to 80 ° C. This temperature may be about 130 ± 1 ° C. Preferably, the reaction of step c) is carried out at a pressure ranging from 20 bar to 1 bar. Advantageously, no particular pressure is applied and the reaction is carried out at atmospheric pressure or in an open system. Preferably, the catalyst used in steps a) and c) of the process is preferably the same. In the process according to the invention, the starting compound (ester) may be present alone or in admixture with other reactants (i.e. other esters). In addition, the starting compound can be used in purified form or in crude form, in particular unrefined, this in particular when the compound is obtained from vegetable oils (eg oleaginous). According to a particular aspect of the invention, step c) of the process, and preferably all steps a) to c) is carried out in the absence of any additive (other than the catalyst) that may have an effect on the reaction coupling of alcohol and / or the production of molecular hydrogen. According to another preferred aspect of the invention, the process does not comprise a drying step and / or degassing starting materials or the reaction medium. Indeed, it has been surprisingly determined that the catalyst (and particularly the catalysts in which Z is HBH3 such as Ru-MACHO-BH, of formula C or la) remains active in the presence of air and traces of water. . It appears indeed that, unexpectedly, these specific reaction conditions make it possible to obtain both a shorter reaction time and the use of a very low catalyst load while preserving a high yield. The fact of being able to overcome the use of additional chemical compounds such as those mentioned above makes it possible to reduce the manufacturing costs, to avoid the problems of corrosion related to their use and therefore to drastically reduce the environmental impact of such processes. This also greatly simplifies the operations of separation and / or purification of the reaction products downstream of the process. According to a particular aspect of the invention, the catalyst is preferably a catalyst of formula 1, in which Z is HBH3 and / or Y is a CO radical and R is phenyl radicals.

According to one aspect of the invention, the catalyst is preferably a catalyst of formula 1 in which the four R radicals are identical. According to a particularly preferred aspect of the invention, the catalyst used is a catalyst of formula 1 in which the Z group is H and the Y group is a phosphine PR '3 where R' is a C 1 -C 12 alkyl or C 6 aryl group. C12, in particular a methyl, ethyl, isopropyl or phenyl group. According to a preferred variant of the invention, the catalyst is the catalyst of Formula 1b (R = 1-Pr, Z = HBH3, Y = CO). According to a preferred variant of the invention, the catalyst is the catalyst of formula la or C (R = Ph, Z = HBH3, Y = CO).

According to a preferred variant of the invention, the catalyst is the catalyst of formula Ic (R = Cy, Z = HBH3, Y = CO). According to a preferred variant of the invention, the catalyst is the catalyst of Formula 6a (R = Ph, Z = H, Y = CO). According to a preferred variant of the invention, the catalyst is the catalyst of Formula 6b (R = isopropyl, Z = H, Y = CO). According to a preferred variant of the invention, the catalyst is the catalyst of formula 6c (R = Cy, Z = H, Y = CO). The invention also relates to the catalysts described in this application as such as well as to their manufacturing processes. In particular, a complex of formula 1c, 1b, 6a and 6c, and a complex of the same formula wherein the carbonyl substituent is substituted by a phosphine is an object of the invention. Their uses in catalytic synthesis processes as well as such methods are also objects of the invention. The invention also relates to an ester obtained directly by the method described above. According to the second embodiment of the invention, the process is a process for the synthesis of cerides.

According to another embodiment it is also possible to use in step a) and c) catalysts of different formulas I. It is also possible to use the alcohol extracted in step b) as a starting material for the synthesis of the second ester.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLE 1 Synthesis of Oleate Oleate (a Ceride) from a Methyl Ester The reaction scheme of the reaction is as follows: ## STR1 ## Methyl Bate -2H 2 Ru Cat o Oleoylate HH. "- N, - u Ru Cal = pe" Y R2 Za: Y = CO; Z = HBH3; R = Ph lb: Y = CO; Z = HBH3; R = 11: K: Y = CO; Z = HBH3; R = Cy Catalyst la (4.5 mg; 7.7 .muol), 1b (3.4 mg; 7.7 iimol) or 1c (4.7 mg, 7.7 μmol) is introduced into an autoclave containing a stir bar. 2.3 g of methyl oleate (7.7 mmol) is introduced via a syringe under an argon atmosphere. The autoclave is then purged three times with a vacuum / hydrogen cycle and then about 20 bar of hydrogen are introduced. The system is heated to 100 ° C with an oil bath and stirred magnetically for 18 hours. Methanol and oleic alcohol are obtained. It is noted that the unsaturations of the fatty chain are not hydrogenated during this process. The selectivities and conversion rates obtained are compiled in Table I: Table I Cat. (0.1 mol%) Selectivity Alcohol Selectivity Oleate Oleyl conversions of oleyl onb 80% 20% 93% lb 76% 24% 92% lc 73% 27% 93% aSelectivity and conversion determined by 1H NMR. b Conversion of methyl oleate.

The reaction medium is transferred under argon using a syringe in a Schlenk tube. The methanol is evaporated under vacuum (20 ° C., 30 min, 1.10-3 mbar). The Schlenk tube is then equipped with a condenser surmounted by a bubbler. The system still containing the catalyst 1a, 1b or 1c dissolved is then heated at 130 ° C for 18h. The concomitant release of gaseous molecular hydrogen is observed and can therefore be used for the synthesis of fatty alcohol. The products obtained are determined by NMR (Bruker Avance NMR Spectrometer, 300 MHz, 5 mm probe) and are compiled in Table II: Table II (0.1 oleate selectivity Oleate Conversion Oleate Selectivity Mol%) oleyl methyl 1a 95% 5% 58% 1b 93% 7% 87% 95% 5% 86% aSelectivity and conversion determined by 1H NMR bConversion of oleic alcohol This transformation makes it possible to obtain cerides directly from the esters This methodology can be generalized to other fatty acid methyl esters (FAMEs) irrespective of the length of the carbon chain of the starting methyl esters: Example 2: Synthesis of Carbonylchlorohydrido [bis (2-diisopropylphosphinoethyl) amino] ruthenium (II) (2b) The reaction scheme of the reaction is as follows: Synthesis: In a Schlenk tube a suspension of carbonylchlorohydrido [tris (triphenylphosphine)] ruthenium ( II) (St rem Chemicals, 0.999 g, 1.04 mmol) and NH (C2H4PiPr2) 2, 3a (0.357 g; 1.17 mmol) in diglyme (10 mL) is placed in an oil bath preheated to 165 ° C and stirred for two hours to give a clear yellow solution. The solution is left for 18h at room temperature to give a precipitate. 10 ml of pentane are added and the suspension cooled at 0 ° C. for 1 hour. The supernatant is then removed and the crystals are washed with diethyl ether (3 x 5 mL) and then dried under reduced pressure to give the desired product as a very pale yellow powder. Yield 64% (0.317 g). 1 H NMR (CD2Cl2, 121.5 MHz): δ. 74.7 ppm. 1H and 31P NMR in agreement with the spectral data of the literature. See: Bertoli, M .; Choualeb, A .; Lough, A. J .; Moore B .; Spasyuk, D .; Gusev D. G. Organometallics, 2011, 30, 3479.

Example 3 Synthesis of Carbonylchlorohydrido [bis (2-dicyclohexylphosphinoethyl) amineuthenium (II) (2c): The reaction scheme of the reaction is as follows: ## STR2 ## - 65 ° C. 19h Synthesis: In a Schlenk tube a suspension of carbonylchlorohydrido [tris (triphenylphosphine)] ruthenium (II) (Strem Chemicals, 1.000 g, 1.05 mmol) and NH (C2H4PCy2) 2, 3b (0.498 g, 1.07 mmol) ) in diglyme (10 mL) is placed in an oil bath preheated to 165 ° C and stirred for 19 hours to give a suspension. The medium is then cooled to room temperature, the supernatant is removed and the precipitate is washed with diethyl ether (3 x 5 mL) and then dried under reduced pressure to give the desired product as a very pale yellow powder. Yield: 78% (0.518 g) 31 P NMR {1 H} (CD 2 Cl 2, 121.5 MHz): δ. 65.6 ppm. 1H and 31P NMR in agreement with the spectral data of the literature.

See: Nielsen, M.; Alberico, E .; Baumann, W .; Drexler H.-J .; Junge, H .; Gladiali, S .; Beller, M. Nature, 2013, (doi: 10.1038 / nature11891) Example 4: Synthesis of Carbonylhydrido (tetrahydroborato) [bis (2-di-ipropylphosphinoethyl) amino] ruthenium (II) (1 b) The reaction scheme of the reaction is as follows Synthesis: In a Schlenk tube under argon flow, a solution of NaBH4 (5 mg, 0.24 mmol) in ethanol (2 mL) is added to a suspension of carbonylchlorohydrido [bis (2-di-i) propylphosphinoethyl) amino] ruthenium (II), 2b (50 mg, 0.08 mmol) in toluene (8mL). The Schlenk tube is then sealed, immersed in an oil bath preheated to 65 ° C and stirred for 2 hours to give an opalescent solution. The solvent is then distilled off under reduced pressure (room temperature, 1.10-3 mbar). The white residue obtained is extracted with dichloromethane (3 × 5 ml) and filtered on sintered glass. The filtrate is then concentrated under reduced pressure (room temperature, 1.10-3 mbar) to give the desired product in the form of a white powder (30 mg, yield: 62%). 1H NMR (CD2Cl2, 300 MHz): 6. 3.90 (broad, 1H, NH); 3.30-3.12 (m; 2H); 2.56-2.44 (m; 2H); 2.30-2.21 (m; 2H); 1.94 - 1.82 (m; 2H); 1.38 (dd, JHP = 16.2 Hz, JHH = 7.5 Hz, 6H); 1.28-1.14 (m, 18H); -1.92 - -2.69 (broad; 4H; RuHBH3); 13.53 (t, JHP = 17.7 Hz, 1H, RuH). 31 P- {1H} (CD2Cl2, 121.5 MHz): δ. 77.7 ppm Example 5 Synthesis of carbonylhydrido (tetrahydroborato) [bis (2-dicyclohexylphosphinoethyl) amineuthenium (/ 1) (1c): The reaction scheme of the reaction is as follows: 3H4. 65: C, 4 h Synthesis: In a Schlenk tube under argon flow, a solution of NaBH4 (48 mg, 1.37 mmol) in ethanol (12 mL) is added to a suspension of carbonylchlorohydrido [bis ( 2-dicyclohexylphosphinoethyl) amino] ruthenium (II), 2c (200 mg, 0.43 mmol) in toluene (16 mL). The Schlenk tube is then sealed, immersed in an oil bath preheated to 65 ° C and stirred for four hours to give an opalescent solution. The solvent is then distilled off under reduced pressure (room temperature, 1.10-3 mbar). The white residue obtained is extracted with dichloromethane (3 × 5 ml) and filtered on sintered glass. The filtrate is then concentrated under reduced pressure (room temperature, 1.10-3 mbar) to give the desired product in the form of a white powder (171 mg, yield: 88%).

1H NMR (CD2Cl2, 300 MHz): δ. 3.85 (broad; 1H; NH); 3.27-3.09 (m; 2H); 2.27-2.10 (m; 8H); 1.96-1.68 (m, 20H); 1.61-1.20 (m, 22H); -2.19-2.50 (broad; 4H; RuHBH3); -13.60 (t, JHp = 17.9Hz; 1H; RuH). H 2 (CD 2 Cl 2, 121.5 MHz): δ. 68.9 ppm. Example 6 Synthesis of carbonyl (dihydrido) [bis (2-diisopropylphosphinoethyl) amineuthenium (II) (6b): Synthesis: In a Schlenk tube a whitish suspension of carbonylhydridoclhoro [bis (2-diisopropylphosphinoethyl) amino] ruthenium (II) 2b (221 mg, 0.468 mmol) in THF (8 mL) is treated with a commercial solution of 1 M NaHBEt3 in toluene (0.45 mL, 0.450 mmol). Then, the medium is left stirring at room temperature under an argon atmosphere for 18 hours to give an opalescent light yellow solution. The solution is then concentrated under reduced pressure to give a yellow solid residue which is dissolved with toluene (8 mL). This new solution is filtered on sintered glass and concentrated under reduced pressure to give a yellow powder. Yield 89% (181 mg) 1 H NMR (C6 D6, 300 MHz): δ. 2.20 - 2.04 (m; 3H); 1.92 - 1.83 (m; 4H); 1.66 - 1.54 (m; 2H); 1.36 - 1.27 (m, 24H); 1.01 - 0.91 (m; 2H); -6.18 - -6.46 (m, 2H, RuH2). 31 P {1 H} NMR (C6D6, 121.5 MHz): δ. 91.1 ppm.

Example 7 Synthesis of carbonyl (dihydrido) [bis (2-dicyclohexyl) phosphinoethylamineuthenium (10 (6c) Synthesis: In a Schlenk tube a whitish suspension of carbonylhydridoclhoro [bis (2-di-cyclohexylphosphinoethyl) amino] ruthenium (II) 2c (63 mg, 0.1 mmol) in THF (2 mL) is treated with a commercial solution of 1M NaHBEt3 in toluene (0.12 mL, 0.12 mmol), then the medium is allowed to stir at room temperature under an argon atmosphere for 18 hours to give an opalescent light yellow solution The solution is then concentrated under reduced pressure to give a yellow solid residue which is dissolved with toluene (2 mL) This new solution is filtered on sintered glass and concentrated under reduced pressure to give a yellow powder Yield 80% (48 mg) 1 H NMR (C6 D6, 300 MHz): δ 2.32 - 2.07 (m, 6H); 1.60 (m, 31H), 1.34 - 1.03 (m, 16H), -6.18 - -6.34 (m, 2H, RuH 2) 31 P {1H} NMR (C6D6, 121, m.p. 5 MHz): Δ 80.4 ppm The invention is not limited to the embodiments presented and other embodiments will become apparent to those skilled in the art.

Claims (11)

REVENDICATIONS1. A process for synthesizing a second ester from a first ester which comprises the following steps: a) bringing a first ester and a catalyst of formula 1 and dihydrogen into the presence of a first alcohol and a second alcohol; b) separating said second alcohol from the reaction medium; c) reacting said first alcohol with the catalyst of formula 1 to obtain a second ester and dihydrogen; and - d) recycling the dihydrogen obtained in step c) by introducing it in step a); said formula 1 being: wherein R are groups, identical or different, selected from the group consisting of cyclohexyl, phenyl, isopropyl and ethyl; where Y is a PR'3 phosphine, a CO radical or a hydrogen atom, R ', which may be identical or different, and are C1-C12 alkyl or C6-C12 aryl groups; and wherein Z is a hydrogen atom, an HBH3 group or a halogen atom. 2. Method according to claim 1, characterized in that step c) of the process is carried out without adding a hydrogen acceptor. 3. Method according to claim 1 or 2, characterized in that the step or steps a) and / or c) of the process is / are carried out without addition of solvent. 4. Method according to claim 3, characterized in that the step or steps a) and / or c) of the process is / are performed without addition of base. 5. Method according to any one of claims 1 to 3, characterized in that Z is not a halogen atom. 6. Process according to any one of claims 1 to 5, characterized in that the R 'group is a methyl, ethyl, isopropyl or phenyl group. 7. Process according to any one of claims 1 to 6, characterized in that R is a cyclohexyl or isopropyl group. 8. Process according to any one of claims 1 to 7, characterized in that said first ester has the following formula: R 1 R 2 where R 1 is an alkyl group of C 2 to C 22, and R 2 is an alkyl group of C 1 to C 6 . 9. Process according to any one of claims 1 to 7, characterized in that said first ester has the following formula: R3 R3 OC) 7 R3 0 0 where R3 are identical or different C8 to C22 alkyl groups. 10. Process according to any one of claims 1 to 9, characterized in that the charge of said catalyst of formula 1 used in step b) is chosen in a range from 500 ppm to 50 ppm. 11. Use of a catalyst of formula 1 in a process as described in any one of claims 1 to 10.